Young attendant, quiet and mature (4)
I believe the answer is:
page
'young attendant' is the definition.
(I've seen this before)
'quiet and mature' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'and' is a charade indicator (letters next to each other).
'mature' becomes 'age' (synonyms).
'p'+'age'='PAGE'
(Other definitions for page that I've seen before include "Side" , "Kind of leaf, or boy in old court" , "errand-boy" , "Young male attendant" , "Section of biography perhaps" .)